src/main.c (Init): chmod /ssd to YAFFS_ROOT_MODE to keep up with RTEM…

…S changes
RTEMS now enforces permissions and RTEMS-YFFS got them wrong before,
making all M1 /ssd file systems created before this change de facto
inaccessible for Flickernoise.
With this commit, we change the permission to the YAFFS default (0777)
when mounting /ssd, transparently migrating all older installations.